The present invention relates generally to methods for heat treating metals and alloys, and more particularly to a heat treating method for producing preselected graded microstructures in nonferrous metals and alloys.
In conventional surface and selective heat treatment methods for ferrous materials, surface layers or other selected regions are typically rapidly heated to the single-phase austenite phase and then rapidly cooled. Temperatures somewhat higher than those typically used for furnace based processes are used to minimize the time required for solutioning the ferrite and carbide phases. Because of the rapid increase in solutioning kinetics with temperature, short austenizing times (seconds) can be used. The treated material has a hardened surface layer with good strength and abrasion resistance and a tough, unhardened ductile core.
Titanium and nickel-base alloys are usually heated in large vacuum, atmosphere or air furnaces, at heat times typically of the order of hours, the final microstructure being uniform throughout the alloy. Graded microstructures in nonferrous materials have heretofore been produced only using a large number of expensive steps including preform design and forging, intermediate furnace heat treatment (beta annealing), finish forging and final (subtransus) furnace heat treatment.
The invention solves or substantially reduces in critical importance problems with prior art processes as just suggested by providing a method for the rapid and controlled surface heating of nonferrous metals and alloys, particularly titanium and nickel base alloys with suitable (usually fine, equiaxed, two-phase) starting microstructures, to produce graded microstructure and corresponding graded properties in semifinished or finished parts with substantially no distortion. The method is based on rapid heating of surface or selective regions of the nonferrous part to temperatures sufficiently high to bring about a phase transformation, such as solution of a second phase, followed by cooling at an appropriate rate to retain the high temperature phase so produced or to transform the selectively heated region to a microstructure substantially different from that of the starting material. By this means, regions containing essentially the starting microstructure, a different heat treated microstructure, and a transition zone may be produced. The invention allows achievement of desired combinations of creep and fatigue resistance, toughness, strength and other properties.
It is therefore a principle object of the invention to provide a heat treatment method for producing graded microstructure in nonferrous metals and alloys.
It is another object of the invention to provide a heat treatment method for titanium and nickel based alloys.
It is yet another object of the invention to provide a heat treatment method for producing graded microstructures in titanium and nickel based alloys for aerospace applications.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.